In cellular networks, such as Long Term Evolution (LTE), a concept of carrier aggregation (CA) is being deployed. CA implies that several (two or more) Component Carriers (CC) can be utilised for communication between a network node and a user equipment (UE). The CCs can be contiguous in the frequency domain or non-contiguous.
The distance between centre frequencies of CCs is called channel spacing of the CCs. All subcarriers in LTE resource blocks are aligned to one a single 15 kHz grid. Moreover, the centre subcarrier is aligned to a 100 kHz grid. This is done in order to simplify UE carrier search and attachment procedures. In order to accommodate both these requirements, channel spacing of CCs need to align to a grid of multiples of 300 kHz.
A small allocated frequency block for a CC, say 5 MHz will, with a 5 MHz channel bandwidth LTE carrier, not have much, if any, frequency space available to be shifted to align to a 300 kHz grid. An alignment to a 300 kHz grid could mean an adjustment by up to 200 kHz, when the band edges conform to even 100 kHz (which is typically the case). This means, in some scenarios, when considering guard bands at the edge of the frequency block, that an operator will not be able to use the non contiguous intra band carrier aggregation at all.
It would be of great benefit if, whenever possible, even small frequency blocks could be used for component carriers in carrier aggregation, while still supporting legacy UEs.